I have a question regarding to the onion skin model in Drosophila melanogaster. I understand why D. melanogaster amplificates its chorion genes etc.I always read that the onion skin model uses an unidirectional ORI. My question: the ORI in the illustrations which i've looked at is always placed in the center of the amplification bubbles. The amplification (so as the replication?) however proceeds into two directions. So why is the origin of replication unidirectional? Are the illstrations wrong or am i missing something? In my opinion one ORI has to be placed either at the left or the right of the "onion skins" / the chorion gene cluster. Am i right?

Sorry, I don't know what you're talking about, I don't know the onion-skin-model, but from what you wrote I would understand, that it goes only on one strand in each point, not like in the case with Okazaki's fragments.

I mean the process which is described below. The paper also says that the ORI lies in the middle of the gene. The illustration looks like the replication went both ways. From my understanding a unidirectional ORI should only start a replication in one and not two directions. So it has to lie either at the right or the left of the amplification bubbles...

"Unlike the amphibian rRNA genes in Example 13.16, the eggshell genes of Drosophila can be amplified without extra chromosomal replication. A large number of follicle cells surround the egg and produce the chorion. The genes encoding the chorionic proteins exist in two clusters (one on the X chromosome and one on an autosome). Only a single copy of each somatic gene is present. A developmentally controlled origin of replication, located within each gene cluster, is programmed to fire 3–6 times during interphase within the 5 h of choriogenesis. The process shown in Fig. 13-1 usually produces a 32- to 64-fold amplification of the eggshell genes."

I cannot access the paper you are talking about and the picture does not explain much. The only thing I can tell from it is that it clashes with your statement: "The paper also says that the ORI lies in the middle of the gene" since it says that "replication origin near the chorion genes"...